Gallium and zinc-loaded ZSM-5 zeolites for the aromatization of propane

Abstract:

ZSM-5 zeolite-based materials with different % XRD crystallinities were synthesized and used as catalysts in acid-catalyzed reactions. The "-hexane cracking reaction was studied under conditions that promoted the monomolecular cracking mechanism whereby a linear relationship was observed between the catalytic activity and the SiO<sub>2</sub>/AI<sub>2</sub>O<sub>3</sub> ratio of the H-ZSM-5 zeolites of high % XRD crystallinity (60-80%). However, with H-ZSM-5-based materials, a non- linear relationship was observed between catalytic activity and % XRD crystallinity. Low activities were observed with samples having XRD crystallinity lower than 30%, and the catalytic activity became more significant only at higher crystallinity levels. Temperature programmed desorption (TPD) of ammonia analysis showed that strong acid sites were absent in the samples with XRD crystallinity lower than 30%. Therefore, our studies showed that alkane conversion reactions have vastly different threshold XRD crystallinity or zeolitic aluminium level requirements before substantial catalytic activity can be observed, as compared with the alkenes, for which it was previously shown that crystallinity levels as low as 2% were sufficient.<br><br> Our results on propane conversion over H-ZSM-5 zeolite-based materials showed a similar non-linear relationship between catalytic activity and % XRD crystallinity. Also in this case, low catalytic activity was observed at XRD crystallinity levels below 30%. We also noted very low catalytic activity for H- ZSM-5 samples with XRD crystallinities higher than 80% but having aluminium content lower than 0.6 - 0.8 wt%. The propane activity appeared to be proportional to the square of the aluminium content.<br><br> The aromatization of propane was studied over H-ZSM-5, Ga- and Zn-loaded ZSM-5 systems. A comparative study of H-ZSM-5, Ga-ZSM-5 and Zn-ZSM-5 catalysts was undertaken involving the ZSM-5-based materials having XRD crystallinities ranging from the substantially amorphous to partially crystalline and their highly crystalline ZSM-5 analogues. Zinc or gallium was introduced into the parent zeolitic structure by means of solid-state ion-exchange or wet impregnation methods. Zinc appeared to enhance both the activity and BTX (benzene, toluene and xylenes) selectivity, relative to the zeolitic parent solid, in contrast to gallium, which affects only BTX selectivity. In addition, optimum activity plus BTX selectivity under the conditions used were found for XRD crystallinities in the range 50-85%. The maximum BTX yields (single pass basis) were lowest for the H-ZSM-5 samples, markedly higher for the Zn-ZSM-5 catalysts, and at an intermediate level for the Ga-ZSM-5 samples.